Output buffer circuit, input buffer circuit and bi-directional buffer circuit for plural voltage systems

Abstract: Bi-directional buffer circuit comprises a second P-channel transistor (102) whose back gate is connected to a pad (11), a third P-channel transistor (108) disposed between a gate of the second P-channel transistor (102) and the pad (11) with its back gate connected to the pad (11), and a first N-channel transistor (103) and a fifth N-channel transistor (204) whose gates are connected to a power source (Vdd), so that an output buffer circuit (100), an input buffer circuit (200) and a bi-directional buffer circuit can be produced without necessity of any additional processing step, wherein merely a single power source is incorporated, and a voltage higher than the source voltage is permitted to be applied to a common bus.

Description: The present invention relates to a semiconductor integrated circuit operable under circumstances where a plurality of supply voltages are coexistent, as in an instance where the integrated circuit is mounted on a board which is so designed as to operate with a power source including a plurality of supply voltages, and/or bearable against live disconnection. In respect of designing a circuit board, there is recently adopted a technique of reducing a supply voltage to, e.g., 3V for some partial devices while applying 5V to operate the other devices. In most cases, the reason is based on an attempt to realize suppression of the entire power consumption or on the specification that the rated supply voltage for some partial devices is lower than 5V. Under such circumstances where mutually different supply voltages are coexistent for the individual operations, there may arise a problem that, when a signal is fed from a 5V device to a lower-voltage device or the devices operated at mutually different voltages are connected to the same bus, a current comes to flow from the 5V device to the lower-voltage device. Such an undesired current flow may cause considerable disadvantages including an increase of the power consumption, latch-up of the devices and, due particularly to the voltage of 5V, breakdown of transistors in the latest miniaturized high-density devices. Fig. 9 typically shows how a 3V-operated device and a 5V-operated device are coexistent on a board while being connected to a common bus, and Fig. 10 is a schematic circuit diagram of a known conventional bi-directional buffer circuit enclosed with a circle in Fig. 9. When a 3V device and a 5V device are connected to a common bus as shown in Fig. 9, there arise the following problems (1) to (3) in the circuit of Fig. 10. (1) Upon input of 5V from an external bus 10 via a pad 11, a parasitic diode 12a for a P-channel transistor 12 in the final output stage is turned on to consequently form a current path (1), whereby a great leakage current is caused to flow therethrough. (2) Since merely 3V is applied to the gate of the P-channel transistor 12, this transistor 12 fails to be completely turned off and is therefore placed in a weak on-state, so that a current path (2) is formed therein also. (3) As 5V is applied to both an N-channel transistor 13 in the final output stage and an N-channel transistor 14 in the initial input stage, a high electric field is generated to eventually arouse some problems relative to the withstand voltage of gate oxide films, hot carriers and so forth, whereby the N-channel transistors 13 and 14 may be broken down. Fig. 11 typically shows conventional means contrived for solving the problems mentioned above. In this diagram, there is included merely its output buffer circuit alone. For the purpose of preventing a leakage current when the supply voltage is 5V with regard to the bus 10 on the board, 5V is applied to the final output stage. Further, the oxide film of the transistor only in the final output stage is processed to be thicker so that this transistor may not be deteriorated even if 5V is applied to the final output stage. In addition to such countermeasure, a level shift circuit for converting a 3V signal into a 5V one is incorporated to achieve complete turn-off of the P-channel transistor in the final output stage, wherein a 3V signal in a 3V system circuit is converted into a 5V signal, which is then supplied to the gate of the P-channel transistor in the final output stage. As a result of the means mentioned, there is contrived an improved circuit system where both a 3V-operated device and a 5V-operated device are rendered connectable to the common bus 10. However, for effecting the measures shown in Fig. 11, it is unavoidable that some processing steps are increased inclusive of forming a thicker oxide film only partially, and a longer time is required for manufacture of a chip to eventually bring about a rise of the production cost. Furthermore, it becomes necessary to supply voltages of two systems, i.e., 3V and 5V, to the devices to consequently induce additional complication of the wiring on the board with a disadvantageous increase in the number of required pins on the chip. In Japanese Patent Laid-open No. Hei 4 (1992)-290008, there is disclosed an off-chip driver circuit which comprises a single pull-up transistor; an output terminal; a voltage source for providing a supply voltage of a predetermined value in an arrangement where the pull-up transistor is disposed between the voltage source and the output terminal; a control transistor connected between the gate electrode of the pull-up transistor and the output terminal, and having a gate electrode connected to a first reference potential point; a bus gate; a predriver circuit coupled via the bus gate to the gate electrode of the pull-up transistor; and means for selectively applying to the output terminal a voltage considerably higher than the aforesaid supply voltage of a predetermined value. However, this known driver circuit also has some disadvantages that, since the substrate (back gate) of the pull-up transistor is biased from the power source via the control transistor, the back gate resistance is rendered high and therefore a latch-up problem is prone to be raised. Further, when a partial circuit board is inserted into and/or extracted from a connector of a main system without cutting power supply thereof, voltage may be applied to the pad without supplying voltage to the output buffer circuit due to disconnection of the connector. Also in this live disconnection case, the same current paths (1) (2) as shown in Fig. 10 cause in prior art output buffer circuit. So, there is a problem in reliability. More specifically, when "H" is inputted to the pad 11 through connector pin under the condition that power supply 3V to the last stage transistor 12 is cut down (off) to 0V, currents (1) (2) as shown in Fig. 10 flow within transistor 12 so that parasitic diode 12a and then transistor 12 turn on. According to the present invention, there is provided an output buffer circuit for outputting a signal to a pad, through which the signal is transferred to an external circuit, comprising: first and second pull-up transistors, connected in series between a power source and said pad respectively, wherein a first signal is input to a gate of the first pull-up transistor, and a back gate of the second pull-up transistor is connected to said pad; characterised by: a gate pull-up transistor having a back gate connected to said pad, and arranged to pull up a gate of said second pull-up transistor when a voltage exceeding that of said power source is applied to said pad. The features set out in the preamble portion of Claim 1 are known from EP-A-0 346 898. An embodiment of the invention can provide an output buffer circuit (as well as bidirectional buffer circuit including the same) which is producible without the necessity of any additional processing step, wherein merely a single power source is incorporated and there is permitted the application of a voltage higher than that of the single power source to a common bus. An embodiment of the present invention can also provide an output buffer circuit operable at a high AC speed, and having superior resistance against occurrence of latch-up. An embodiment of the present invention can further provide an output buffer circuit capable of withstanding live disconnection, which can prevent the current paths (1) and (2) mentioned above when the partial circuit board is inserted into and/or extracted from the connector of the main system. Thus, a pad of the output buffer circuit may be connected to a connector for inserting and/or extracting the partial circuit board into or from the main system. In embodiments of the present invention, an output buffer circuit for outputting an "H" (high) level, an "L" (low) level or a high impedance to. a pad, via which a signal is transferred from or to an external circuit, comprises: (1-1) a successive series connection of a first P-channel transistor and a second P-channel transistor between the power source and the pad in this order, wherein a back gate of the first P-channel transistor is connected to the power source and a gate thereof is connected to a first signal input terminal to which a first signal is inputted, the first signal being transited to an "L" level at the time of outputting an "H" level to the pad, or transited to an "H" level at the time of outputting an "L" level or a high impedance to the pad, and a back gate of the second P-channel transistor is connected to the pad; (1-2) a successive series connection of a first N-channel transistor and a second N-channel transistor between the pad and the ground in this order, wherein a gate of the first N-channel transistor is connected to the power source or the first signal input terminal, and a gate of the second N-channel transistor is connected to a second signal input terminal to which a second signal is inputted, the second signal being transited to an "L" level at the time of outputting an "H" level or a high impedance to the pad, or transited to an "H" level at the time of outputting an "L" level to the pad; (1-3) a successive series connection of a third N-channel transistor and a fourth N-channel transistor between the first signal input terminal and the gate of the second P-channel transistor in this order, wherein a gate of the third N-channel transistor is connected to an enable signal input terminal to which an enable signal is inputted, the enable signal being transited to an "H" level at the time of outputting an "H" level or an "L" level to the pad, or transited to an "L" level at the time of outputting a high impedance to the pad, and a gate of the fourth N-channel transistor is connected to either the power source or a third signal input terminal to which a third signal is inputted, the third signal having an inverted logic with respect to the second signal; and (1-4) a third P-channel transistor connected between the pad and the gate of the second P-channel transistor, wherein a back gate of the third P-channel transistor is connected to the pad, and a gate thereof is connected to either the power source or the third signal input terminal. In another embodiment, the output buffer circuit of the present invention is included in a bidirectional buffer circuit together with an input buffer circuit for receiving an "H" level or an "L" level signal from an external circuit to an internal circuit through the same pad as used by the output buffer circuit, the input buffer circuit comprising: (2-1) an inverter; (2-2) a fifth N-channel transistor disposed between the pad and the input terminal of the inverter with its gate connected to the power source; and (2-3) a feedback circuit for raising the input terminal potential of the inverter to the power source potential at the time of inputting an "H" level to the pad. In a buffer circuit of the present invention, mutual connection between the gate of each transistor and the power source or the ground may be either direct or indirect through an impedance element such as a resistor. In an output buffer circuit embodying the present invention, the following measures are adopted with regard to the aforementioned problems (1) to (3). First, the problem (1) relative to turn-on of the parasitic diode is solved by providing the second P-channel transistor whose back gate is connected to the pad as described in (1-1). Regarding the next problem (2) that the P-channel transistor in the final output stage fails to be completely turned off, the third P-channel transistor described in (1-4) is provided to serve as a pull-up transistor for pulling up the gate of the second P-channel transistor, so that 5V applied to the pad is further applied via the third P-channel transistor to the gate of the second P-channel transistor, which is thereby turned off completely. Further, in regard of the problem (3) that a high electric field is induced due to the application of a high voltage such as 5V to the N-channel transistor, the provision of the first N-channel transistor described in (1-2) is effective to disperse the applied high voltage to the first N-channel transistor and the second N-channel transistor, thereby preventing generation of any high electric field that exceeds the withstand voltage of one N-channel transistor. Further, an output buffer circuit according to an embodiment of the invention for outputting an "H" level, an "L" level or a high impedance to a pad via which a signal is transferred from or to an external circuit, comprises: a plurality of pull-up transistors connected in series to each other between a power source and the pad, wherein a first signal is inputted to a gate of the pull-up transistor disposed proximate to the power source, and a back gate of the pull-up transistor disposed proximate to the pad is connected to the pad; a plurality of pull-down transistors connected in series to each other between the pad and a ground, wherein a second signal is inputted to a gate of the pull-down transistor disposed proximate to the ground; an enable transistor and a control transistor connected in series to each other between gates of the plurality of pull-up transistors, wherein an enable signal is inputted to a gate of the enable transistor, and a third signal having an inverse logic with respect to the second signal is inputted to a gate of the control transistor; and a gate pull-up transistor disposed between the control transistor and the pad. Furthermore, the third signal may be inputted to a gate of the gate pull-up transistor. Further, a gate of the gate pull-up transistor may be connected to the power source. Furthermore, a gate of the pull-down transistor disposed proximate to the pad may be connected to the power source. Further, the first signal may be inputted to a gate of the pull-down transistor disposed proximate to the pad. Furthermore, a mutual junction of the plurality of pull-up transistors may be connected to a mutual junction of the plurality of pull-down transistors. Further, a back gate of the pull-up transistor disposed proximate to the power source may be connected thereto. Thus, according to the present invention, improvements are attained in the output buffer circuit and the bi-directional buffer circuit producible without the necessity of any change or addition of the processing steps, wherein merely a single power source is required and still the entire problems (1) to (3) mentioned with reference to Fig. 10 are completely solved. Further, in case of the live disconnection, when "H" is inputted to the pad 11 under the condition that power supply is cut down (off) and only signal pins are connected, no current flows within the transistors 101 and 102. Because, the back gate of the transistor 102 is connected to the pad 11 by the transistor 108 being "ON" state so that the parasitic diode 12a is not forward direction and it does not turned on. Further, gate of the transistor 102 is charged to a potential of the pad 11 by the transistor 108 so that the transistor 102 is made to be completely "OFF" state. In other word, the case of the live disconnection can be regarded as extreme case (internal voltage = 0V, external voltage = 5V) of plural voltage system. These and other aspects, features and advantages of the present invention will become more apparent upon a reading of the following description and appended drawings. The preferred embodiments will be described with reference to the drawings, wherein like elements have been denoted throughout the figures with like reference numerals, and wherein: Fig. 1 is a circuit diagram showing a first embodiment of the bi-directional buffer circuit according to the present invention; Fig. 2 is a circuit diagram similar to Fig. 1, showing the states of individual elements when an "H" (high) level (3V) signal is outputted to a pad; Fig. 3 is a circuit diagram similar to Fig. 1, showing the states of the individual elements when an "L"(low) level (0V) signal is outputted to the pad; Fig. 4 is a circuit diagram similar to Fig. 1, showing the state where a high impedance is outputted to the pad from the output buffer circuit and an "H" level (5V) is applied to the pad from an external circuit; Fig. 5 is a circuit diagram similar to Fig. 1, showing the state where a high impedance is outputted to the pad from the output buffer circuit and an "L" level (0V) is applied to the pad from the external circuit; Fig. 6 is a circuit diagram showing a second embodiment of the bi-directional buffer circuit according to the present invention; Fig. 7 is a circuit diagram showing a third embodiment of the bi-directional buffer circuit according to the present invention; Fig. 8 is a circuit diagram similar to the third embodiment of Fig. 7 rewritten with partial modification of its output buffer circuit; Fig. 9 typically shows how a 3V-operated device and a 5V-operated device are coexistent with mutual connection to a common bus on a board; Fig. 10 is a circuit diagram showing a conventional bi-directional buffer circuit of the prior art enclosed with a circle in Fig. 9; and Fig. 11 typically shows the known measures adopted in the prior art. Hereinafter the preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Fig. 1 is a circuit diagram of a first embodiment which represents the bi-directional buffer circuit according to the present invention. An output buffer circuit 100 constituting the bi-directional buffer circuit of Fig. 1 serves for outputting an "H" level, an "L" level or a high impedance to a pad 11. In the output buffer circuit 100, there is a successive series connection of a first P-channel transistor 101 and a second P-channel transistor 102 between a 3V power source Vdd and the pad 11 in this order, wherein a back gate of the first P-channel transistor 101 is connected to the power source Vdd while a back gate of the second P-channel transistor 102 is connected to the pad 11. Also in the output buffer circuit 100, there is a successive series connection of a first N-channel transistor 103 and a second N-channel transistor 104 between the pad 11 and a ground GND in this order. A gate of the first P-channel transistor 101 is connected to an output terminal of a first inverter 105, and a signal D1 is inputted to the first inverter 105. In this embodiment, an output terminal 121 of the first inverter 105, i.e., a gate terminal of the first P-channel transistor 101, corresponds to a first signal input terminal in the present invention, and therefore the signal D1 is the one obtained by inverting the logic of the first signal defined in this invention. The gate of the second P-channel transistor 102 is connected to an output terminal 121 of the first inverter 105 via a third N-channel transistor 106 and a fourth N-channel transistor 107 which are connected in series between the output terminal 121 of the first inverter 105 and the gate of the second P-channel transistor 102. Further a third P-channel transistor 108, whose back gate is connected to the pad 11, is connected between the gate of the second P-channel transistor 102 and the pad 11. An enable signal EN is inputted to the gate of the third N-channel transistor 106. In this embodiment, the gate terminal of the third N-channel transistor 106 corresponds to an enable signal input terminal. A gate of the fourth N-channel transistor 107 and a gate of the third P-channel transistor 108 are both connected to an input terminal 123 of a second inverter 109. And a signal D2 is inputted to the second inverter 109. A gate of the first N-channel transistor 103 is connected to the power source Vdd, while a gate of the second N-channel transistor 104 is connected to an output terminal 122 of the second inverter 109. In this embodiment, the output terminal 122 of the second inverter 109, i.e., the gate terminal of the second N-channel transistor 104, corresponds to a second signal input terminal defined in this invention; and the input terminal 123 of the second inverter 109, i.e., each gate terminal of the fourth N-channel transistor 107 and the third P-channel transistor 108, corresponds to a third signal input terminal defined in this invention. Therefore the signal D2 corresponds to the third signal defined in the present invention, and the signal obtained by inverting the third signal in the second inverter 109 corresponds to the second signal defined in the present invention. An input buffer circuit 200 constituting the bi-directional buffer circuit of Fig. 1 comprises an inverter 201 which consists of a pair of P-channel transistor 202 and N-channel transistor 203 connected in series between a power source Vdd and a ground GND, a fifth N-channel transistor 204 disposed between the pad 11 and an input terminal of the inverter 201 with its gate connected to the power source Vdd, a feedback P-channel transistor 205 disposed between the power source Vdd and the input terminal of the inverter 201 with its gate connected to an output terminal of the inverter 201, and a third inverter 206 whose input terminal is connected to the output terminal of the inverter 201. Fig. 2 is a circuit diagram similar to Fig. 1, showing the states of individual elements when an "H" level (3V) signal is outputted to the pad 11. In outputting an "H" level to the pad 11, both of the signals D1 and D2 are transited to an "H" level, while the enable signal EN is held at an "H" level. When the signals D1 and D2 are transited to an "H" level in the state where the enable signal EN is held at an "H" level, an "L" level is applied to the gate of the first P-channel transistor 101, so that the first P-channel transistor 101 is turned on and then a parasitic diode 102a of the second P-channel transistor 102 is bulk-charged to thereby start transition of the pad 11 to an "H" level. Since the third N-channel transistor 106 and the fourth N-channel transistor 107 are both turned on, the second P-channel transistor 102 is also turned on thereafter, whereby an "H" level is outputted to the pad 11 via the first P-channel transistor 101 and the second P-channel transistor 102. And simultaneously an "L" level is applied to the gate of the second N-channel transistor 104 to thereby turn off the same. In this stage of the operation, an "H" level is applied to the input buffer circuit 200 as well, and due to the application of 3V from the power source Vdd to the gate of the fifth N-channel transistor 204, the input terminal of the inverter 201 receives a voltage (Vdd - Vth) which is lower than the source voltage Vdd by a threshold voltage Vth, e.g., 2.2V when Vth = 0.8V. Consequently the potential at the output terminal of the inverter 201 is lowered to thereby turn on the P-channel transistor 205, so that the source voltage Vdd = 3V is applied via the P-channel transistor 205 to the input terminal of the inverter 201. As a result, the output terminal of the inverter 201 is completely turned to an "L" level, and this signal is supplied via the third inverter 206, which then transits the signal D3 to an "H" level. The above operation is performed in the input buffer circuit 200 in response to the "H" level output to the pad 11, but normally the signal D3 is not used at the time of outputting the signal from the output buffer circuit 100 to the pad 11. As described, the bi-directional buffer circuit of this embodiment performs a satisfactory operation without any problem when outputting an "H" level signal to the pad 11. Fig. 3 is a circuit diagram similar to Fig. 1, showing the states of the individual elements when an "L" level (0V) signal is outputted to the pad 11. In outputting an "L" level to the pad 11, both of the signals D1 and D2 are transited to an "L" level. Meanwhile the enable signal EN is still held at an "H" level at this time as well. When the signals D1 and D2 have both been transited to an "L" level in the state where the enable signal EN is held at an "H" level, the second N-channel transistor 104 is turned on. Since the first N-channel transistor 103 is also in its on-state at this time, the pad 11 and the ground GND are connected to each other via the first and second N-channel transistors 103 and 104, whereby an "L" level (0V) is outputted to the pad 11. In this stage of the operation, an "H" level is applied to the gate of the first P-channel transistor 101 to consequently turn off the same, whereby the gate potential of the second P-channel transistor 102 Is rendered indefinite. However, as its back gate is connected to the pad 11, the potential is changed to 0V so that the second P-channel transistor 102 is also turned off. And simultaneously, an "L" level is applied to the input terminal of the inverter 201 in the input buffer circuit 200, thereby turning the output terminal of the inverter 201 to an "H" level while turning the output signal D3 of the third inverter to an "L" level. Thus, the bi-directional buffer circuit of this embodiment performs a satisfactory operation without any problem also when outputting an "L" level signal to the pad 11. Fig. 4 is a circuit diagram similar to Fig. 1, showing the state where a high impedance is outputted from the output buffer circuit 100 to the pad 11 and an "H" level (5V) is applied from an external circuit to the pad 11. In this stage of the operation, an "L" level is inputted as the enable signal EN, while an "L" level and an "H" level are inputted as the signals D1 and D2 respectively. At this time, an "H" level is applied to the gate of the first P-channel transistor 101, which is thereby turned off. And simultaneously therewith, 3V of the signal D2 is applied to the gate of the third P-channel transistor 108 while 5V of the pad 11 is applied to the back gate thereof, so that a current denoted by reference numeral (2) in Fig. 10 flows in the third P-channel transistor 108 to thereby cause application of 5V to the gate of the second P-channel transistor 102. Consequently the second P-channel transistor 102 is placed completely in its off-state. In this stage of the operation, the fourth N-channel transistor 107 is in its on-state as an "H" level (3V) of the signal D2 is applied to the gate thereof, but since it is an N-channel transistor, the potential at the junction of the third N-channel transistor 106 and the fourth N-channel transistor 107 is 2.2V for example, so that any high electric field exceeding the withstand voltage is not generated in the fourth N-channel transistor 107 either. Further the potential difference between the source and the drain of the third N-channel transistor 106 is suppressed to be as low as 0.8V. Since an "L" level is applied simultaneously therewith to the gate of the second N-channel transistor 104, this transistor 104 is turned off and then 5V applied to the pad 11 is divided to the first N-channel transistor 103 and the second N-channel transistor 104, so that any high electric field exceeding the withstand voltage is not generated in the first N-channel transistor 103 or the second N-channel transistor 104 either. A voltage of 5V applied to the pad 11 is subsequently applied to the fifth N-channel transistor 204 in the state where 3V is being applied to the gate of this transistor 204, so that 2.2V is applied to the input terminal of the inverter 201. As a result, merely 2.8V is applied between the source and the drain of the fifth N-channel transistor 204, and consequently any high electric field exceeding the withstand voltage is not generated therein. Subsequently, as mentioned, 3V is applied to the input terminal of the inverter 201 via the feedback P-channel transistor 205, and then the output of the inverter 201 is turned to an "L" level while the output signal D3 of the third inverter 206 is turned to an "H" level, and thereafter the "H"-level signal D3 thus obtained is fed to the inside. As described above, when a high impedance is outputted from the output buffer circuit 100 in this manner and 5V is applied to the pad 11 from an external circuit, there arises none of the problems (1) to (3) described with reference to Fig. 10, and the 5V "H"-level signal applied to the pad 11 is converted to a 3V "H"-level signal and then is fed to the inside. Fig. 5 is a circuit diagram similar to Fig. 1, showing the state where a high impedance is outputted from the output buffer circuit 100 to the pad 11 and an "L" level (0V) is applied from the external circuit to the pad 11. In this case, each transistor operates in such a manner as shown in Fig. 5, and the "L"-level signal (0V) applied to the pad 11 is fed to the inside without causing any of the problems (1) to (3) described with reference to Fig. 10. Thus, according to the embodiment shown in Fig. 1, an improved bi-directional buffer circuit can be realized without the necessity of any modification or addition in the processing steps, wherein merely a single 3V power source is incorporated and application of an external voltage of 5V is permitted. In this embodiment, the back gate (substrate) of the first P-channel transistor 101, which serves as a power supply side pull-up transistor with its source electrode connected to the power source Vdd, is connected directly to the power source Vdd, and the second P-channel transistor 102 is existent between the first P-channel transistor 101 and the pad 11, so that it becomes possible to prevent occurrence of a latch-up trouble. Fig. 6 is a circuit diagram of a second embodiment which represents the bi-directional buffer circuit of the present invention. A description will now be given below only on the difference from the aforementioned first embodiment shown in Fig. 1. A gate of a first N-channel transistor 103 in the second embodiment is connected to an output terminal 121 (first signal input terminal) of a first inverter 105. Meanwhile a gate of a fourth N-channel transistor 107 is connected to an input terminal 123 of a second inverter 109, and a gate of a third P-channel transistor 108 is connected to a power source Vdd. The connection of each gate of such first N-channel transistor 103 and third P-channel transistor 108 constituting an output buffer circuit 100 may be changed individually as shown in Fig. 1 or Fig. 6. A gate of a feedback P-channel transistor 205 in an input buffer circuit 200 of the second embodiment is connected to an output terminal of an inverter 207, and an input terminal of the inverter 207 is connected to an input terminal of an inverter 201. In this manner, the feedback circuit for raising the input terminal potential of the inverter 201 from 2.2V to 3V may be formed into a variety of configurations without being limited to the aforementioned example alone. Fig. 7 is a circuit diagram of a third embodiment which represents the bi-directional buffer circuit of the present invention. A description will now be given below only on the difference from the aforementioned first embodiment shown in Fig. 1. In the third embodiment of Fig. 7, a junction 110 of a first P-channel transistor 101 and a second P-channel transistor 102 is connected to a junction 111 of a first N-channel transistor 103 and a second N-channel transistor 104. Fig. 8 is a circuit diagram similar to the third embodiment of Fig. 7 rewritten with partial modification of its output buffer circuit. Due to the mutual connection of the two junctions 110 and 111, the second P-channel transistor 102 and the first N-channel transistor 103 are connected in parallel to each other as shown in Fig. 8. It may be possible in both of the first and second embodiments as well to drive fast an external load connected to the pad 11 by turning on the parasitic diode at the time of a level transition. Meanwhile, in the third embodiment where the second P-channel transistor 102 and the first N-channel transistor 103 are in parallel connection, a greater current is permitted to flow in comparison with the first and second embodiments when the signal of the pad 11 is changed from an "L" level to an "H" level or from an "H" level to an "L" level, whereby the external load can be driven at a higher speed. For example, a Bi-directional buffer circuit comprises a second P-channel transistor (102) whose back gate is connected to a pad (11), a third P-channel transistor (108) disposed between a gate of the second P-channel transistor (102) and the pad (11) with its back gate connected to the pad (11), and a first N-channel transistor (103) and a fifth N-channel transistor (204) whose gates are connected to a power source (Vdd), so that an output buffer circuit (100), an input buffer circuit (200) and a bi-directional buffer circuit can be produced without necessity of any additional processing step, wherein merely a single power source is incorporated, and a voltage higher than the source voltage is permitted to be applied to a common bus. Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the scope of the invention as defined in the claims.

Claims: An output buffer circuit (100) for outputting a signal to a pad (11), through which the signal is transferred to an external circuit, comprising: first and second pull-up transistors (101, 102), connected in series between a power source (Vdd) and said pad (11) respectively, wherein a first signal is input to a gate of the first pull-up transistor (101), and a back gate of the second pull-up transistor (102) is connected to said pad; characterised by: a gate pull-up transistor (108) having a back gate connected to said pad (11), and arranged to pull up a gate of said second pull-up transistor (102) when a voltage exceeding that of said power source (Vdd) is applied to said pad (11). The output buffer circuit according to claim 1, wherein said pad (11) is connected to a connector which may be disconnected without cutting the power supply. The output buffer circuit (100) according to claim 1, for outputting an "H" level, an "L" level or a high impedance output to the pad (11), further comprising: first and second pull-down transistors (103, 104), the pad (11), the first pull-down transistor (103), the second pull-down transistor (104) and the ground (GND) being connected in series in this order, and wherein a second signal is input to a gate of the second pull-down transistor (104); and an enable transistor (106) and a control transistor connected in series with each other between gates of said first and second pull-up transistors (101, 102), wherein an enable signal (EN) is input to a gate of said enable transistor (106), and a third signal having an inverse logic with respect to said second signal is input to a gate of said control transistor (107). The output buffer circuit (100) according to claim 3, wherein said third signal is input to a gate of said gate pull-up transistor (108). The output buffer circuit (100) according to claim 3, wherein a gate of said gate pull-up transistor (108) is connected to said power source (Vdd). The output buffer circuit (100) according to claim 3, 4 or 5, wherein a gate of said first pull-down transistor (103) is connected to said power source (Vdd). The output buffer circuit (100) according to claim 3, 4 or 5, wherein said first signal is input to a gate of said first pull-down transistor (103). The output buffer circuit (100) according to any of claims 3 to 7, wherein a junction (110) between said first and second pull-up transistors (101, 102) is directly connected to a junction (111) between said first and second pull-down transistors (103, 104). The output buffer circuit (100) according to any of claims 3 to 8, wherein a back gate of said first pull-up transistor (101) is connected to said power source (Vdd). The output buffer circuit (100) according to claim 3, wherein: the first pull-up transistor (101) is a first P-channel transistor, and has a back gate connected to said power source and a gate connected to a first signal input terminal (121) to which a first signal is input, the first signal making a transition to an "L" level at the time of outputting an "H" level to said pad, or to an "H" level at the time of outputting an "L" level or a high impedance to said pad (11); the second pull-up transistor (102) is a second P-channel transistor; the first and second pull-down transistors are first and second N-channel transistors respectively, a gate of said first N-channel transistor (103) being connected to said power source (Vdd) or said first signal input terminal (121), and a gate of said second N-channel transistor (104) being connected to a second signal input terminal (122) to which a second signal is input, said second signal making a transition to an "L" level at the time of outputting an "H" level or a high impedance to said pad (11), or to an "H" level at the time of outputting an "L" level to said pad (11); the enable transistor (106) is a third N-channel transistor and the control transistor (107) is a fourth N-channel transistor, said enable signal making a transition to an "H" level at the time of outputting an "H" level or an "L" level to said pad (11) or making a transition to an "L" level at the time of outputting a high impedance to said pad (11), and a gate of said fourth N-channel transistor (107) is connected to a third signal input terminal (123) to which said third signal is input; and the gate pull-up transistor (108) is a third P-channel transistor connected between said pad (11) and the gate of said second P-channel transistor (102), and a gate thereof is connected to either said power source (Vdd) or said third signal input terminal (123). The output buffer circuit (100) according to claim 10, wherein the gate of said third P-channel transistor (108) is connected to said power source (Vdd) or ground (GND) via an impedance element. The output buffer circuit (100) according to claim 10 or 11, wherein a junction (110) of said first P-channel transistor (101) and said second P-channel transistor (102) is connected to a junction (111) of said first N-channel transistor (103) and said second N-channel transistor (104), whereby said second P-channel transistor (102) and said first N-channel transistor (103) are connected in parallel to each other. A bi-directional buffer circuit including said output buffer circuit (100) according to any of claims 1 to 12 and an input buffer circuit (200) for feeding to an internal circuit an "H" level or an "L" level signal from an external circuit input to the same pad (11) to which said output buffer circuit supplies its output signal, the input buffer circuit comprising: an inverter (201); an N-channel transistor (204) disposed between said pad (11) and an input terminal of said inverter (201) with its gate connected to a power source (Vdd); and a feedback circuit for raising the input terminal potential of said inverter (201) to the power source potential when an "H" level signal is input to said pad (11).

Inventors: KEIDA HISAYA, KOBAYASHI KENICHIRO, KEIDA, HISAYA, KOBAYASHI, KENICHIRO, KOBAYASHI, KENICHIRO, C/O TOKYO HEAD OFFICE
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1994
Main classification: H03K19
IPC codes: H03K19, G05F1, H03K5, G05F3, H01L27, H01L21